Introduction Half of the world's population are exposed to household air pollution from biomass fuels, which are increasingly recognised as a global risk factor for impaired respiratory health. Data from case-control studies has associated Tuberculosis (TB) with biomass fuel exposure. We hypothesised that particulate matter (PM) within alveolar macrophages of individuals exposed to biomass smoke detrimentally affects macrophage function by: (A) Reduced phagocytosis of TB, leading to increased infectivity and (B) Impaired macrophage killing of TB, leading to increased susceptibility to disease. This study assessed the ability of wood smoke exposed human monocyte-derived macrophages (MDMs) to phagocytose and kill TB in vitro.

Methods MDMs, from buffy coats, were cultured in vitro for 12 days and then exposed to suspensions of wood smoke PM (Malawian (MW) and Norwegian (NW) wood, 10 μg/ml or 50 μg/ml) for 8 h. The PM loaded cells were then challenged with TB. Phagocytosis was assessed by manually counting infected macrophages, using fluorescent microscopy and by quantitative culture of day 1 supernatant and macrophage lysate. Remaining cells were maintained in culture for 7 days before lysis and quantitative culture to assess intracellular growth.

Results Microscopy data showed a reduced proportion of infected macrophages in the high dose NW smoke group (Abstract P64 figure 1A), and fewer TB bacilli per macrophage in the two MW smoke groups (Abstract P64 figure 1B). Quantitative culture of day 1 supernatant and cell lysate showed no difference between the groups (Abstract P64 figure 1C). Quantitative culture of day 7 lysate showed reduced Mycobacteria loads in the high dose NW smoke group (Abstract P64 figure 1D).

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Abstract P64 Figure 1

The effect of wood smoke on monocyte-derived macrophages to phagocytose and kill Mycobacterium tuberculosis: (A) Effects of wood smoke on macrophage phagocytosis of TB (assessed by microscopy). (B) Mean numbers of bacilli per infected macrophage. (C) Mean time to positivity for culture of day 1 supernatants. (D) Calculated Mycobacterial load of day 7 lysates (log scale).

Discussion We have developed an in vitro model to assess the interaction between TB and wood smoke exposed macrophages. Phagocytic data does not fully support increased TB infectivity in biomass smoke exposed individuals. Quantitative culture data does not demonstrate a difference in the ability of wood smoke exposed MDMs to kill TB; high dose NW smoke led to a reduced intra-macrophage growth of TB probably as a result of the toxic effect of PM on Mycobacteria growth. Modifications of this model are required to provide mechanistic evidence of an association of TB with wood smoke exposure.